Earth or space observation satellites collect data including Earth images, land exploration data, weather observation data, maritime surveillance data, or forest monitoring data. Earth or space observation satellites are commonly low Earth orbit (LEO) satellites and orbit the Earth at an altitude in the range of 300 to 1,000 km above the Earth's surface. In 2017, approximately 1071 LEO satellites are in orbit and the number of LEO satellites to be launched is expected to increase.
LEO satellites require a communication link with a ground station for telemetry, tracking, and command (TT&C) and for transmitting Earth or space observation data to one or more ground stations. When used for Earth or space observation, LEO satellites can (1) capture and store Earth Observation (EO) data in on-board memory as the LEO satellite orbits the Earth; and (2) transmit the stored EO data when the LEO satellite has line-of-sight visibility with a ground station during a process known as “store and forward”. The duration of time that the LEO satellite has line-of-sight visibility with the target ground station depends on the orbiting altitude and inclination of the LEO satellite and the latitude of the target ground station. As LEO satellites orbit at relatively low altitudes from the Earth's surface, the duration of time when the LEO satellite has line-of-sight visibility with a target ground station is typically in the range of 5 to 15 minutes per orbit of the Earth. When the LEO satellite has line-of-sight visibility with the target ground station, the LEO satellite can transmit data and telemetry to the target ground station and receive commands from the target ground station. In some scenarios, because the amount of EO data or TT&C data can be large, the duration of time required to transmit EO data to the target ground station can exceed the duration of time that the LEO satellite may have line-of-sight visibility with the target ground station. Any EO data or TT&C data not transmitted by the LEO satellite to the target ground station may continue to be stored in on-board memory and transmitted to the target ground station when the LEO satellite re-establishes line-of-sight visibility on a subsequent orbit of the Earth, thus delaying EO data transmission to the target ground station. When the EO data has a defined shelf life (e.g., weather observation data), delaying transmission of the EO data from the LEO satellite to the target ground station may render the weather observation data to become stale or out-of-date. Further, if TT&C data transmission and reception at the LEO satellite is delayed, a network operations center at the target ground station may be unable to resolve issues or otherwise control the LEO satellite in near real-time or in a timely way.
In some scenarios, to facilitate large amounts of EO and TT&C data transfer to network operations centers or target ground stations, several successively spaced ground terminals along the Earth's surface that corresponds to the trajectory of a given LEO satellite may be provided. However, building and operating a large number of ground terminals can be costly.
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